1. Technical Field
The present invention relates to a hard particle, a wear-resistant iron-base sintered alloy, and a method of manufacturing the same. Further, the invention relates to a valve seat formed by the sintered alloy, which can be suitably used in gas engines employing gases such as, in particular, CNG (compressed natural gas) or LPG (liquefied petroleum gas).
2. Background Art
JP Patent Publication (Kokai) No. 9-242516 (Patent Document 1) discloses a wear-resistant sintered alloy used in valve seats. The alloy is manufactured by compacting a powder comprising a base material component and cobalt-base hard particles. The base material component comprises 0.5% to 1.5% C by weight, 2.0% to 20.0% at least one element selected from the group consisting of Ni, Co and Mo by weight, with the remainder being Fe, against 100% of the powder. The cobalt-base hard particles comprise 26% to 50% by weight of the powder. The green compact is molded and then sintered at high temperatures to form the wear-resistant sintered alloy. In this example, the cobalt-base hard particles are made of an intermetallic compound with Vicker's hardness (Hv) of 500 or more, containing Co as the principal component and heat-resistant, corrosion-resistant elements (such as Mo, Cr and Ni). In this sintered alloy, the oxide layer formation on the hard particles and the base material is insufficient. As a result, adhesion tends to occur due to the relative sliding movements of the metals. Further, there is not much dispersion between the hard particles and the base material during sintering, resulting in insufficient joint strength, so that the hard particles tend to fall away. The alloy, therefore, does not have a sufficient wear resistance.
JP Patent Publication (Kokai) No. 2001-181807 (Patent Document 2) discloses a wear-resistant sintered alloy similarly used in valve seats. The alloy as a whole contains 4% to 30% Mo by mass, 0.2% to 3% C by mass, 1% to 20% Ni by mass, 0.5% to 12% Mn by mass, with the remainder being unavoidable impurities and Fe. The base material consists of 0.2% to 5% C by mass, 0.1% to 12% Mn by mass, with the remainder being unavoidable impurities and Fe. Hard particles consist of 20% to 70% Mo by mass, 0.5% to 3% C by mass, 5% to 40% Ni by mass, 1% to 20% Mn by mass, with the remainder being unavoidable impurities and Fe. The hard particles are dispersed in the base material in an areal ratio of 10% to 60%.
In this sintered alloy, the amount of dispersion of Mn contained in the hard particles into the base material of the sintered alloy is large, so that the adhesion between the hard particles and the base material can be improved. Thus, the retainability of the hard particles is improved, the density of the sintered alloy can be increased, and the hardness and wear resistance of the alloy can be increased. Further, the hard particles do not contain Cr as an active element, thus facilitating the formation of an oxide layer of Mo on the hard particles. The Mo oxide layer functions as a solid lubricant, thus providing the hard particles with lubricity, in addition to hardness and wear resistance. As a result, the alloy according to this publication proves highly effective as the material for valve seats or valve guides in CNG- or LNG-fueled engines, in which the solid lubricity in the slide range tends to be low as compared with that in the valve system of gasoline engines.
Patent Document 1: JP Patent Publication (Kokai) No. 9-242516 A (1997)
Patent Document 2: JP Patent Publication (Kokai) No. 2001-181807
In the course of experiments conducted on various materials for valve seats and valve guides to be used in engines, particularly those fueled with CNG or LNG, the inventors arrived at the conclusion that, although the wear-resistant sintered alloy disclosed in JP Patent Publication (Kokai) No. 2001-181807 has high wear resistance, a sintered alloy is needed that has higher wear resistance if higher engine performance is to be obtained. It is therefore an object of the invention to provide a hard particle, a wear-resistant iron-base sintered alloy, a method of manufacturing the wear-resistant iron-base sintered alloy, and a valve seat wherein an oxide layer of the hard particle can be easily formed and high wear resistance can be obtained.